Current devices that monitor the relative alignment of the two eyes with each other use eye-tracking methods to determine the accuracy of fixation of each eye separately on a specified fixation point. The relative stability of the alignment of the two eyes with each other is thereby inferred by comparison of the accuracy of fixation of the two eyes separately.
Numerous eye-tracking devices and methods are known to the art. Specifically useful, especially with children, are those methods that determine the gaze point of an eye from a distance, without requiring apparatus worn on the head of the subject. Many such methods infer the direction of gaze from the position of the pupil in a video image as the eye looks in one direction or the other. Such pupil-tracking methods require the head to be still and require the obtaining of multiple calibration points within the visual scene, involving the recording of pupil positions versus respective test points in the visual scene, for accurate estimation of the gaze point of the eye during subsequent eye tracking and recording.
Other eye-tracking devices and methods use not only the position of the pupil in a video image, but also use the position of the corneal light reflection of a small source of light. The image of the corneal light reflection is a virtual image approximately 1 mm posterior to the pupil in the human eye, so that both the pupil and the corneal light reflection can be imaged sharply by the recording camera. When the eye moves in one direction, both the pupil and the corneal light reflection move in that same direction but at different speeds, with the corneal light reflection moving only about half as fast as the movement of the pupil. Therefore the position of the corneal light reflection, with respect to the position of the pupil, with appropriate calibration, yields an estimate of the gaze point, in relation to the positions of the camera and the small source of light, which is reasonably independent of head position. If the small source of light is placed conjugate to the aperture of the camera via a beam splitter, the video image of the eye has a bright pupil, because the eye's optics return the reflected light from the retina back towards the source, where it enters the camera aperture, yielding the bright pupil in the image. If the small source of light is placed eccentric enough to the camera aperture, the pupil remains dark, although the corneal light reflection of the source of light is still present, somewhat eccentric. Both bright-pupil and dark-pupil methods have been used successfully for eye tracking.
The primary remaining disadvantage of eye tracking devices and methods that use images of the pupil, and/or images of lights reflected by the cornea, to estimate the gaze point, is the necessity to calibrate the device for each individual eye, given the variability of eye size, corneal curvature, and pupil positions in the population.
Other devices and methods exist for detecting when an eye is looking in a given direction. The fovea is that part of the retina where vision is most acute, and this is the part of the retina that is aimed at the object of regard during fixation. The nerve fibers in the retina radiate out from the fovea like the spokes of a bicycle wheel. These nerve fibers have a small amount of “form” birefringence that changes the nature of polarized light that passes through them, with the type and amount of polarization change varying with the angular direction of the nerve fibers. Polarized incident light that is reflected from the back of the eye passes through these nerve fibers twice, and the resulting change in the polarization of the double-pass light can be detected by sensors near the source of the polarized light. By scanning a spot of light about in a specified manner on the retina, a spatial birefringence-produced polarization signature can be obtained of that patch of nerve fibers encountered by the scan. Specifically, if a circular scan of the spot of light is centered exactly on the fovea, polarization changes can be detected in the double-pass light that occur at exactly twice the frequency of the scan, as described in U.S. Pat. No. 6,027,216. A high level of this double-frequency signal thus detects eye fixation on a spot in the center of the circular scan of light. This method of detecting eye fixation in a given direction has been termed “retinal birefringence scanning.”
Detection of eye fixation via retinal birefringence scanning does not require calibration in specific directions of gaze as do the video-based pupil-tracking methods, because retinal birefringence scanning detects the anatomic fovea directly by sensing centration of the circular scan on the radial nerve fibers emanating from the fovea. But using retinal birefringence scanning for eye tracking, rather than simply for eye fixation detection, becomes complicated because the polarization signatures of many areas of the nerve fibers away from the fovea are neither unique nor of high amplitude.
There are still other methods for tracking the positions of the eyes. These include tracking the positions of only the corneal light reflections from the eyes, tracking the positions of the corneal/scleral junctions of the eyes, tracking the positions of anatomic features of the eyes using optical coherence tomography, and tracking the positions of the eyes using scleral search coil recordings.
Recently it has been discovered that eyes with amblyopia (decreased vision in an eye caused by misalignment or defocus in early life), typically have a small amount of misalignment with the other eye when viewing a small fixation target. U.S. Pat. No. 7,959,292 B2 describes a binocular retinal birefringence scanning device which, by detecting such small amounts of misalignment on a fixation target, can identify children (or adults) with amblyopia. Even more recently it has been discovered that eyes with amblyopia do not fixate steadily on a small target, either under monocular or binocular conditions. [See González E G, et al. Invest Ophthalmol Vis Sci. 2012; 53(9):5386-94, and Subramanian V, Jost R M, Birch E E. Invest Ophthalmol Vis Sci. 2013; 54(3):1998-2003.] It therefore appears that there is not a constant misalignment of the amblyopic eye, but rather a varying misalignment.
The eyes normally move tightly together in various directions of gaze, with this “conjugacy” of the movements of the two eyes regulated and maintained by the normal binocular vision system. When the eyes have roughly equal vision and are working perfectly well together (“fusing” peripherally and centrally), the brain continually senses if and when the eyes begin to become misaligned, via impending double vision, and makes fine adjustments to the signals to the eye muscles to keep the eyes aligned. Over time, with repetition of these signals occurring in various directions of gaze, “vergence adaptation” causes a “map” to be established in the brain, a map of how much each of the 12 eye muscles should be stimulated to maintain alignment of the two eyes with each other in each direction of gaze and at each distance from the individual. The result is that the eyes move tightly together when binocular vision is normal, both remaining fixated tightly on the object of regard. Even when one eye is covered, its movement behind the cover is quite conjugate with the movement of the fixing eye—again, when binocular vision is normal.
This normal conjugacy of the movements of the two eyes with one another has been documented for decades by binocular eye tracking when subjects are instructed to look quickly from one specific point to another (such a quick eye movement is called a “saccade”). To be sure, horizontal binocular saccades have been shown to be disconjugate when one eye is amblyopic [Maxwell G F, Lemij H G, Collewijn. Conjugacy of saccades in deep amblyopia. Invest Ophthalmol Vis Sci. 1995; 36:2514-2522], and also horizontally disconjugate when strabismus is present (misalignment of the two eyes) [Kapoula Z, Bucci M P, Eggert T, Garraud L. Impairment of the binocular coordination of saccades in strabismus. Vision Res. 1997; 37:2757-2766]. Furthermore, in the presence of strabismus, saccades have been shown to be disconjugate in direction as well as in amplitude [Walton W W G, Ono S, Mustari M. Vertical and oblique saccade disconjugacy in strabismus. Invest Ophthalmol Vis Sci. 2014; 55:275-290].
Thus deficits of binocular function (strabismus, amblyopia, monocular blindness, etc.) cause disconjugacy of eye movements and disconjugacy of the moment-to-moment positions of the two eyes with respect to one another. Indeed, we have experimentally confirmed that, in the presence of deficits of binocular function (occurring with strabismus, amblyopia, monocular blindness, traumatic brain injury, inebriation, fatigue, etc.), the positions of the two eyes vary with respect to one another during ordinary viewing. In the reverse, detecting variability of the alignment of the two eyes with each other (detecting “disconjugate” alignment over time) can serve as a means to screen for deficits of binocular function. The eye-tracking or fixation-detecting methods described above may be used for such a purpose, but each such method is subject to significant limitations, especially with infants and young children. For example, the pupil-tracking methods and corneal light reflex/pupil-tracking methods all require gaze calibration for proper functioning, with such calibration difficult if not impossible with infants and young children. The binocular retinal birefringence scanning method only detects the presence or absence of fixation of each eye separately; it does not easily detect the amount of misalignment.
Again, to date, devices that monitor the relative alignment of the two eyes with each other use eye-tracking methods to determine the accuracy of fixation of each eye separately on a specified fixation point. The relative stability of the alignment of the two eyes with each other has thereby been inferred by comparison of the accuracy of fixation of the two eyes separately. In one instance the relative horizontal positions of one eye with respect to the other have been calculated from binocular recordings during a fixation task [Raveendran R N, Babu R J, Hess R F, Bobier W R. Transient improvements in fixational stability in strabismic amblyopes following bifoveal fixation and reduced interocular suppression. Ophthalmic Physiol Opt 2014. doi: 10.1111/opo.12119]. In this case, however, this “relative position” technique was to determine how long a bifoveal fixation condition persisted after an initial 10 seconds of bifoveal fixation. The technique used a specific fixation target and was not used for a measurement of the overall variability of the conjugacy of the two eye's positions over time. Likewise, it was not used for the identification of deficits of binocular function.
Note that all of the methods discussed above have required the subject to gaze at, or follow, a small fixation target, a task that is not reliably performed by infants or small children, who comprise the primary population that needs to be screened for deficits of binocular function. Detecting these deficits early in life enables more timely and more effective therapy for lifelong improvement. A method and device are therefore needed to detect variability in the moment-to-moment alignment of the two eyes with each other, to thereby detect deficits of binocular function, without requiring specified gaze on small fixation targets.